A novel miniature catheter-mounted electrode capable of reliable and real-time pH measurements in blood and tissue is proposed. The principle of the pH sensing electrode is based on the reversible hydrogenelectrode, which is the ultimate standard for pH determination. Difficulties associated with glass pH electrodes for in vivo measurements can, therefore, be alleviated. This pH sensing electrode, when used in conjunction with an indwelling pCO2 and pO2 sensing electrode, will provide real-time physiological information necessary for ventilation management and acid base balance maintenance in intensive care, operating rooms, and research laboratories.